The present invention relates to a process for preparing 3,5,5-trimethylcyclohexa-2-en-1,4-dione which per se is highly valued as a perfume for foods, fragrances and tobaccos and is useful as an intermediate for the production of drugs and perfumes, e.g. carotnoids and vitamin E.
Heretofore, various processes have been proposed for preparing 3,5,5-trimethylcyclohexa-2-en-1,4-dione. According to the process disclosed in West German Pat. No. 2,365,546, 3,5,5-trimethylcyclohexa-2-en-1-one (hereinafter referred to as ".alpha.-isophorone") is oxidized with chromium oxide in a mixed solvent of acetic acid and acetic anhydride to thereby prepare 3,5,5-trimethylcyclohexa-2-en-1,4-dione in a yield of about 50% relative to .alpha.-isophorone. This process is disadvantageous in that the percentage conversion and yield are low; it is required to use a large excess of chromium oxide relative to .alpha.-isophorone, thus causing the danger of explosion; the production cost is high; and much labor is required for the disposal of chromium waste. According to West German Pat. No. 2,457,158, .alpha.-isophorone is oxidized in vapor phase in the presence of a vanadium catalyst to thereby prepare 3,5,5-trimethylcyclohexa-2-en-1,4-dione in a yield of about 30% relative to .alpha.-isophorone. This process is disadvantageous in that the percentage conversion and yield are low and 5,5-dimethyl-3-formylcyclohexa-2-en-1,4-dione is byproduced in about the same amount as 3,5,5-trimethylcyclohexa-2-en-1,4-dione. According to West German Pat. No. 2,459,148, .alpha.-isophorone is oxidized in liquid phase in the presence of an acetylacetonato iron or cobalt catalyst or a rhodium (I) tristriphenylphosphine chloride catalyst to thereby prepare 3,5,5-trimethylcyclohexa-2-en-1,4-dione in an yield of about 30% relative to .alpha.-isophorone. This process is disadvantageous in that selectivity and yield are poor. In Japanese Patent Laid-Open No. 191645/1986, 3,5,5-trimethylcyclohexa-2-en-1,4-dione is prepared in a yield of about 50% by oxidized .alpha.-isophorone in the presence of an alkali metal or an aromatic amine and phosphomolybdic acid or silicomolybdic acid. This process is disadvantageous in that the percentage conversion and yield are low and .alpha.-isophorone, which is difficult to be separated from 3,5,5-trimethylcyclohexa-2-en-1,4-dione, is contained in about the same amount in the reaction product. In French Pat. No. 2,254,730, 3,5,5-trimethylcyclohexa-3-en- 1-one (hereinafter referred to as ".beta.-isophorone") is oxidized in an alcohol solvent in the presence of a tertiary amine and copper (II) salt of pyridine to thereby prepare 3,5,5-trimethylcyclohexa-2-en-1,4-dione in a yield of about 30% relative to .beta.-isophorone. This process is disadvantageous in that the yield is low and a polymer of .beta.-isophorone is byproduced in a fairly large amount. Further, in West German Pat. No. 2,457,157, .alpha.-isophorone is oxidized over a period of about 35 hours in the presence of an acetylacetonate complex catalyst which is derived from a transition metal such as vanadium, chromium, copper, manganese, iron, cobalt, or nickel to thereby prepare 3,5,5-trimethylcyclohexa-2-en-1,4-dione in a maximum yield of 55% relative to .beta.-isophorone. This process is disadvantageous in that the reaction time is long and yield is low.
It is the object to the present invention to provide a process for preparing 3,5,5-trimethylcyclohexa-2-en-1,4-dione industrially advantageously which process is free of the above-mentioned disadvantages or drawbacks of the prior art.